Wrap up

Entropy is a measure of disorder. The second law of thermodynamics states that entropy increases in all real processes. A century and half ago, Kelvin expressed it as ‘no engine can completely convert heat to work’ while Clausius observed that ‘heat can be moved from a cold to a hot body only by doing work’. These observations lead to limits on the efficiency of power plants and refrigerators. And, Carnot showed that the ideal efficiency of a cyclic system could be expressed in terms of the temperatures between which the system operates.

See the Key Relationships PDF at the bottom of the page to reflect on what you have covered this week.

What’s next?

Next week we will look at how the second law limits the processes that occur spontaneously and how nature gets around this apparent limit. The third law of thermodynamics will also be introduced and you will be given a glimpse beyond absolute zero.